A photo ionization detector (hereinafter “PID”) is known as a conventional device for measuring the concentration of a measuring-subject gas. The PID carried out its measurement in the following manner. The measuring-subject gas is led to a pair of voltage applying electrodes inside the PID, where ultraviolet with a short wavelength is emitted onto the gas to ionize it. Ions of the gas are then captured by the voltage applying electrodes and a detection current correlated with the gas concentration is measured. The measured current value is then converted into a gas concentration value. In this manner, the PID measures the concentration of a volatile organic substance, etc.
A PID 50 shown in FIG. 10 includes a detection chamber 52 to and from which a measuring-subject gas is led and discharged, a pair of electrodes 54 disposed in the detection chamber 52, a voltage applying circuit 56 that applies a DC voltage to the electrodes 54, a UV lamp unit 58 that emits ultraviolet onto the measuring-subject fluid (FL) (measuring-subject gas) inside the detection chamber 54, a measuring circuit 60 that measures a current flowing through the electrodes 54, and a computing device 62 that carries out computation for converting a current value into the concentration value of the measuring-subject gas.
According to the PID 50 as a conventional PID, the measuring-subject gas is led into the detection chamber 52, is exposed to the ultraviolet from the UV lamp unit 58, and is discharged from the detection chamber 52. The measuring-subject gas led into the detection chamber 52 is ionized by the ultraviolet from the UV lamp unit 58 disposed on the side wall of the detection chamber 52, and ions or electrons making up the ionized gas are attracted to and captured by the electrodes 54 supplied with the DC voltage inside the detection chamber 52. Hence a current is generated in the circuit connected to the electrodes 54.
The measuring circuit 60 measures the generated current, and the computing device 62 multiplies the measured current value by a factor specified for each substance (substance making up the measuring-subject gas) to calculate the concentration of the measuring-subject gas. Such a PID is sensitive to many kinds of measuring-subject gases and therefore is an effective means for measuring a measuring-subject gas. Being constructed into a compact form, the PID allows easy measurement of a measuring-subject gas and is appreciated as an extremely useful device.
However, because the current flows in a fixed direction in the above conventional PID, a contamination material, such as an insulting material, is deposited on the metal surface of one of the two electrodes supplied with a DC voltage during a long period of service. This contamination prevents the ionized measuring-subject gas from reaching the electrodes, thus preventing the generation of a current flow. As a result, measuring an accurate current value becomes impossible, which means a drop in the PID's sensitivity to the measuring-subject gas. It has been pointed out as a problem that when the metal electrode is covered with the contamination material, the PID loses its measurement sensitivity and becomes incapable of exerting its functions.
For example, patent document 1 discloses a photoionization detector for detecting a volatile organic compound, which detector includes detecting electrodes that detect a volatile organic compound in a measuring-subject fluid, a voltage/current applying unit that applies an AC voltage or alternating current to the detecting electrodes, a UV lamp that emits ultraviolet onto the measuring-subject fluid to ionize the volatile organic compound in the measuring-subject fluid, an exciting circuit that excites the UV lamp, and a measuring unit that measures a current or voltage flowing through or applied across the detecting electrodes.
According to this photoionization detector, an AC voltage or alternating current is applied to the detecting electrodes. For this reason, in addition to an ion current generated at the electrodes, a different current is also generated as the detecting electrodes work as a capacitor. The ionized volatile organic compound changes the dielectric loss of the detecting electrodes, which results in a change in the volume of electric charges accumulated in the capacitor. Hence a change in a current value caused by a change in the volume of electric charges can be measured. Therefore, even if a contamination material is deposited on the detecting electrodes, applying an alternating current to the electrodes prevents the loss of the measurement sensitivity. When an AC voltage or alternating current is applied to two detecting electrodes, even if a contamination material is deposited on one detecting electrode, the other electrode can capture the ionized volatile organic compound. This prevents a drop in the measurement sensitivity.